The most commonly used temperature scales are the centigrade scale and the Fahrenheit scale. Both of the two scales are used in some countries, while either of the two is used in some other countries. Such being the case, temperature measuring apparatus are desired, as the case may be, to have a function of displaying measured temperatures selectively in both the two scales.
A Fahrenheit temperature value t.sub.F (.degree.F.) is related to a centigrade temperature value t.sub.C (.degree.C.) through an equation: ##EQU1## which shows that the freezing point 0.degree. C. and boiling point 100.degree. C. of water under a pressure of 1 atmosphere correspond to 32.degree. F. and 212.degree. F., respectively.
In a temperature measuring apparatus devised so as to analog-display on an analog meter a temperature measured, for instance, by making use of the temperature-dependent characteristic of a semiconductor device such as a transistor, the measured temperature can easily be read out in both the centigrade and the Fahrenheit scales by graduating the analog meter in both the centigrade and the Fahrenheit scales. However, in case of digitally displaying the measured temperatures, the apparatus not only necessiates, on the assumption that a temperature sensor used therein outputs a signal in proportion to a centigrade temperature value, an A-D converting means for digitally displaying the centigrade temperature value itself, but also needs, for a digital display of a corresponding Fahrenheit temperature value, either a means for digitalizing through A-D conversion a signal which is made proportional to a corresponding Fahrenheit temperature value in advance by performing an analog arithmetic operation on the output from the temperature sensor in accordance with Eq. (1), or a means for performing, in accordance Eq. (1), a digital arithmetic operation on a signal quantized in advance by A-D converting the output from the temperature sensor. The present invention employs the former system in which an analog arithmetic operation on the sensor output is followed by an A-D conversion. Relating to the system a prior art is described in the following on reference to FIG. 4:
In the figure, a temperature sensing circuit 2, which uses a transistor 1 as a temperature sensor, outputs on an output terminal 3 a voltage e.sub.C =kt.sub.C which is proportional to a centigrade temperature t.sub.C. An A-D converter 5, which has input terminals REF.sup.+ and IN.sup.+ for receiving respectively a reference voltage e.sub.r and signal voltage e.sub.s, outputs a digital signal proportional to e.sub.s /e.sub.r, the ratio of the signal voltage e.sub.s to the reference voltage e.sub.r. The ratio e.sub.s /e.sub.r is displayed by a display unit 6. The A-D converter 5 is supplied at its reference voltage input terminal REF.sup.+ with the reference voltage e.sub.r obtained by dividing with the resistor 7 a stabilized positive voltage E.sub.a being supplied from a terminal 8. Further, resistors 10, 11, 12 and an operational amplifier 13 constitute an analog arithmetic operation circuit, which derives according to Eq. (1) a voltage e.sub.F proportional to a Fahrenheit temperature t.sub.F from a voltage e.sub.C (=kt.sub.C) outputted to the terminal 3 in proportion to the centigrade temperature t.sub.C. Either of the two voltages e.sub.C and e.sub.F is selectively inputted to the A-D converter 5 through a switch 4.
In such a circuit constitution as described above, the voltage e.sub.C =kt.sub.C proportional to the centigrade temperature is inputted to the input terminal IN.sup.+ of the A-D converter 5 with the switch 4 turned to a contact a. On the other hand, the reference voltage e.sub.r supplied to the reference voltage input terminal REF.sup.+ of the A-D converter rom the voltage dividing resistor 7 is set at a value of 10.sup.n times the sensitivity k(=e.sub.C /t.sub.c) of the temperature sensing circuit 2, n being selected to be 2, for example, in case three figures of a temperature value is to be displayed. Namely, if k=1 mV/.degree.C., e.sub.r =100 mV. Therefore, a display output C.sub.x from the A-D converter shows a centigrade temperature t.sub.C given by an equation: ##EQU2## where a coefficient 100 is to determine the position of decimal point, making a displayed temperature value be 100.degree. C. when e.sub.C =e.sub.r.
Next, for displaying a Fahrenheit temperature value with the switch 4 turned to a contact b, the resistors 10 and 11 must be coordinated so as to make the operational amplifier 13 output a voltage e.sub.F corresponding to a Fahrenheit temperature t.sub.F in accordance with an equation: ##EQU3## which corresponds to Eq. (1). For the purpose, first the resistor 10 is adjusted, with a stabilized negative voltage -E.sub.d supplied thereto through a terminal 9, so that e.sub.F may be equal to 32k when t.sub.C =0, and secondly the resistor 11 is adjusted so that e.sub.F may be equal to 212k corresponding to 212.degree. F. when t.sub.C =100.degree. C. Such adjustments procedures make Eq. (3) valid in the circuit, and therefore, an output F.sub.x from the A-D converter shows a Fahrenheit temperature t.sub.F given by an equation: ##EQU4##
According to this method, the voltage e.sub.F which is obtained as a voltage corresponding to a Fahrenheit temperature through an analog arithmetic operation performed on an analog signal kt.sub.C outputted from the temperature sensing circuit 2 is digitalized by the A-D converter 5 having its reference voltage kept at the same reference voltage e.sub.r that is used for displaying a centigrade temperature, and then displayed digitally.
However, in case of a transistor being used as a temperature sensor in the above method, the sensitivity of the sensor is as low as 2 mV/.degree.C. for the centigrade temperature and 1 mV/.degree.C. for the Fahrenheit temperature, and therefore, a possible drift error of about .+-.0.5 mV arising in the operational amplifier 13 causes a large quantization error of about .+-.1.degree. F. in the Fahrenheit temperature value. Moreover, in case the Fahrenheit temperatures are to be displayed at a resolving power of 0.1.degree. F., a drift error of .+-.50 .mu.V results in a quantization error of .+-.0.1.degree. F. or more. Such errors are not encountered in case of displaying the centigrade temperature values free from an arithmetic operation, but encountered only in case of displaying the Fahrenheit temperature values.
As is understood from the above description, the prior art method of displaying temperature values selectively in the centigrade scale and in the Fahrenheit scale needs an operational amplifier to convert a centigrade temperature value to a corresponding Fahrenheit value, and moreover, the operational amplifier must disadvantageously be of high precision to suppress the error due to the amplifier to a negligibly small value.